In known manner, and as shown in FIG. 1, a drive train 1 comprises an engine unit 2, a variable-speed transmission 4, and a set of drive wheels 6.
The drive train 1 further comprises various transmission elements, in particular an engine outlet shaft 8 transmitting rotary motion from the outlet of the engine unit 2 to the variable-speed transmission 4, and a wheel shaft 9 transmitting rotary motion from the outlet of the variable-speed transmission 4 to the set of drive wheels 6, via transmission elements that are not shown but that are well known in the state of the art.
In order to avoid overburdening the description below, the engine unit is referred to as the “engine”, it being understood that the engine unit 2 could be constituted by a fuel-burning engine alone, or equally well by other types of engine or motor, or indeed by an association of drive units, for example a fuel-burning engine and an electric motor for a hybrid vehicle.
Similarly, the set of drive wheels 6 is represented by a single wheel, it being understood that as a general rule the set of drive wheels 6 comprises two wheels or four wheels.
Elements of the transmission that have no bearing on setting out the invention are not shown in the diagram of FIG. 1.
The variable-speed transmission 4 is adapted to vary the ratio of the speeds of rotation of the wheel shaft 9 and the engine outlet shaft 8 on a continuous basis, where said ratio is directly associated by a continuous function with the gear ratio L, i.e. the ratio of the vehicle speed V divided by the speed of rotation ω of the engine outlet shaft 8, or the engine speed.
Control of engine speed and of the variable-speed transmission 4 is provided by a function in a computer 11 (e.g. a computer associated with the engine, or a “gearbox” computer, or some other computer on board the vehicle). The computer receives a certain amount of information relating to the operation of the vehicle, and in particular of the drive train and of the acceleration control as actuated by the driver. In response, it issues two control signals C2 and C4 respectively for the engine 2 and for the variable-speed transmission 4, respectively representative of an engine torque setpoint and of a rotary speed ratio setpoint.
The vehicle has a certain number of sensors and/or computation modules serving to estimate values for a set of variables corresponding to the information to be supplied to the computer 11 in order to control the engine 2 and the variable-speed transmission 4. In particular, the vehicle may be provided with a sensor for sensing the position of the accelerator pedal, and adapted to provided the computer 11 with an estimate of the value for the acceleration control variable P1, e.g. in terms of a percentage travel of the acceleration pedal relative to its total stroke.
The vehicle also has a vehicle speed sensor which provides the computer 11 with an estimated value for the speed V of the vehicle.
Means for estimating the value of the speed of rotation ω of the engine outlet shaft 8 are also provided, enabling the corresponding information to be supplied to the computer 11.
Methods are known in the state of the art for controlling a continuously-variable drive train, in which a unit time interval ti is defined, and in which the following steps are performed at each instant corresponding to each unit time interval:                estimating the value of an acceleration control variable;        estimating the value of the vehicle speed;        estimating the value of the speed of rotation of the engine outlet shaft; and        controlling the speed of rotation of the engine outlet shaft (or the gear ratio of the variable-speed transmission) as a function of said estimated values.        
The control strategies used in known control methods sometimes lead to performance of excellent quality, both in terms of optimizing energy consumption and in terms of converting the driver's intention into vehicle speed and wheel torque. Such control strategies are presented in the form of prerecorded maps.
Unfortunately, known methods produce sensations that are poorly accepted by drivers, in particular they produce a sensation of skidding due to the relative changes between engine speed and vehicle speed. It is also found that there are frequent and large variations in engine speed for small variations in the position of the accelerator pedal.
An extremely severe defect of continuously-variable drive trains operated using the above-mentioned methods, consists in unacceptable noise stemming from the above-mentioned drawbacks and due both to the noise levels reached and also to the difference between the “expectations” of users and the noise generated by the engine.